1. Technical Field of the Invention
The present invention relates to an emission of an electromagnetic wave (frequency thereof is equal to or more than 0.01 [THz], and equal to or less than 100 [THz]) (such as a terahertz wave (frequency thereof is equal to or more than 0.03 [THz], and equal to or less than 10 [THz]), for example).
2. Related Art
A broadband terahertz wave generation method by means of a difference-frequency terahertz wave generation using the Cherenkov radiation has conventionally been proposed (refer to FIG. 2 of a Non-patent Document 1, and FIG. 1 of a Patent Document 1). Exciting light from a light source (KTP-OPO excited by Nd-YAG laser, for example) outputting two wavelengths is made incident to a nonlinear crystal (MgO-doped LN crystal, for example) according to FIG. 2 of the Non-patent Document 1. The incident exciting light having two wavelengths induces nonlinear polarization in the MgO-doped LN crystal. If the MgO-doped LN crystal satisfies the condition (nTHz>nopt) of the Cherenkov radiation, a spherical wave which takes a maximal value at each position corresponding to the twice of the coherence length is generated. This spherical wave has wave fronts aligned in a direction of a radiation angle θ satisfying the following relationship, and the terahertz waves are emitted in this direction. In the equation, nopt denotes a refractive index in the exciting light waveband of the MgO-doped LN crystal, and nTHz is a refractive index in the terahertz waveband of the MgO-doped LN crystal.cos θ=nopt/nTHz     [Non-patent Document 1] K. Suizu, K. Koketsu, T. Shibuya, T. Tsutsui, T. Akiba, and K. Kawase, “Extremely frequency-widened terahertz wave generation using Cherenkov-type radiation,” Opt. Express 17(8), 2009, pp. 6676-6681.    [Patent Document 1] Japanese Patent Application Laid-open No. 2010-204488